I have brand-new gentoo installed and during the installation I used USB-tethering to Android-device, it worked pretty smooth: ifconfig - find usb0 - net-setup.
But after installation network isn't working that way.

It would appear that the kernel is not configured to support the wifi interface card.
boot the install media, if it is minimal install cd/usb or sysresccd/usb you can run lsusb or lspci -k to identify the usb or pci wifi interface card.
to get lsusb and lspci as part of the installed system, boot the install media, mount the gentoo partitions, and enter the chroot. Then